This invention generally relates to the art of filament winding composite structures and, particularly, to a method of fabricating a removable mandrel for use in filament winding containers, such as pressure vessels, chemical or water storage tanks, rocket motor cases or any tubular structure which may or may not be closed. Tubular structures may be cylindrical, rectangular, triangular, ellipsoidal, square or other polygonal shapes. Such containers or tubular structures are filament wound on mandrels.
Various types of vessels, such as pressure vessels, rocket casings, and other tubular structures and the like, are fabricated with wall structures of filament wound composite materials. In other words, the thickness of the walls is built up by winding filaments associated with an appropriate resin, whereafter the assembly is cured to form what is called a xe2x80x9cfilament composite structurexe2x80x9d. The filaments may be of glass, graphite or like material. The thickness of the structural walls are built up by winding hoop or helical layers in a desired pattern. In some instances, an impermeable liner or bladder may be used inside the filament composite structure. In some instance, a core or foam layer or thickness may be sandwiched between filament wound layers. A structural inner skin or layer a structural outer skin or layer may be separated by a hollow space that may be vacuumed to reduce heat transfer between the inner and outer skins. The core may be made of lightweight material such as a honeycomb core, closed foam, balsa wood or the like. Various fittings, such as a polar boss at a closed end of the vessel, may be integrated in the composite structure and held in place by the filament windings. A manway may be installed along the cylindrical surface of the structure with no openings at the domes except discharging faucets and the like.
Filament wound vessels of the character described above structure on a mandrel. However, the invention equally applies to hand layed-up vessels or the like. The mandrel and the resulting filament wound vessel typically has one or more dome-shaped closed ends. For instance, a filament wound vessel may have a generally cylindrical portion extending a major length of the vessel corresponding to a cylindrical mandrel about which the vessel is wound. The ends of the mandrel are generally ellipsoidal to form opposite dome-shaped closed ends which may or may not have various fittings, such as polar bosses. The cylindrical portion of the vessel is formed by hoop and longitudinal filament windings. The longitudinal windings may be wound using either helical or polar patterns and extend into the domes at each end of the cylindrical section. After the filament composite structure of the vessel is built-up on the mandrel, the entire assembly is cured and the vessel is removed from the mandrel.
Problems continue to plague fabrication processes as described above, particularly in removing the mandrel or mandrel tooling. Segmented metal mandrels have been used with some success. A segmented mandrel is broken down and removed through an opening in the vessel (i.e., from the inside-out). Unfortunately, segmented mandrels are extremely expensive, very labor intensive and sacrifice tolerance repeatability. Water soluble sand mandrels have been effective for large pressure vessels and rocket motor casings, but the binder in the sand is limited to low temperature cures. In addition, dimensional repeatability of sand surfaces can be a variable or requires extremely expensive tooling.
The present invention is directed to solving this myriad of problems by a simple fabrication process which affords easy removal of the mandrel used in fabricating filament wound containers. It should be understood herein that the term xe2x80x9ccontainerxe2x80x9d is intended to include closed vessels such as pressure vessels and the like, enumerated above, as well as any tubular or spherical structures which in one way or another xe2x80x9ccontainsxe2x80x9d a medium within the confines thereof, whether the container be closed or not.
An object, therefore, of the invention is to provide a new and improved method of fabricating a removable mandrel for use in filament winding a container.
In the exemplary embodiment of the invention, the method includes the steps of providing and inflating an internal bladder. A dry three-dimensional (3-D) fabric layer of a given thickness is layed-up about the inflated bladder. An external vacuum/pressure bag is installed about the fabric layer. The dry fabric layer then is impregnated with a liquid soluble resin between the internal bladder and the external bag. The resin is cured to rigidify the 3-D fabric layer to form a mandrel structure. A container then can be wound on the mandrel formed by the cured and rigidified fabric layer, and the resin subsequently is washed out to remove the fabric.
As disclosed herein, the internal bladder is deflated and removed from inside the mandrel before the mandrel is used to filament wind the container. The bladder is provided with an elongated cylindrical configuration having opposite hemispherical ends. The bladder is inflated through a pressure port at one end thereof.
The dry 3-D fabric layer is layed-up as a thickness of woven fabric, such as of glass fibers. The dry 3-D fabric layer is layed-up in sections to form a desired configuration of the container. For instance, like the bladder, the sections of the dry 3-D fabric layer may be layed-up to form an elongated cylindrical configuration with opposite hemispherical ends.
According to one aspect of the invention, the dry 3-D fabric layer is impregnated by the resin under pressure applied between the internal bladder and the external bag. According to a preferred embodiment of the invention, the dry 3-D fabric layer is impregnated by applying alternating pressure and vacuum between the internal bladder and the external bag. The alternating pressure and vacuum are applied at spaced locations of the bladder and bag. For instance, with the elongated configuration, the alternating pressure and vacuum may be applied at the opposite hemispherical ends of the pressure/vacuum bag.
Preferably, the resin is water soluble so that, after curing, the resin can be dissolved and washed away. The 3-D fabric then can be removed, dried and reused as a subsequent mandrel.
Other features of the invention include the step of applying a release film to the outside of the internal bladder before the dry fabric layer is layed-up thereabout. This facilitates removal of the internal bladder after the mandrel is formed. The external vacuum/pressure bag is removed before a container is filament wound about the mandrel.